1. Field of the Invention
The present invention relates to radio communication apparatuses and amplifiers (radio frequency power modules) incorporated in radio communication apparatuses and, more particularly, to a technique which is advantageously applied to techniques capable of improving amplification efficiency in a low power mode.
2. Description of the Prior Art
An amplifier (RF power module) including a multiplicity of stages of MOSFETs, GaAs MESFETs or the like is incorporated in an output stage at the transmission end of a transmitter of a radio communication apparatus (mobile communication apparatus) such as a mobile phone or portable telephone.
In general, a portable telephone has a system configuration which enables communication with its output varied in adaptation to the environment in accordance with a power level instruction signal from a base station so as not to cause interruption with other portable telephones.
Radio frequency power amplifiers (RF power amplifier circuits) are discussed on pages 115-126 of "Nikkei Electronics" Jan. 27 1997, issued by Nikkei BP. This article discusses standard systems of 900 MHz band cellular type portable telephones in the United States and the GSM system in Europe. This article also discusses output control systems and it states "a widely used output control method is to vary the magnitude of an input signal to the final stage of a transmitter using a programmable attenuator with the gain of the final stage kept constant.
The same article also says "users regard the capability of a portable telephone to communicate a remote base station as important as the life of the battery regardless of the system of the same. All standards for cellular type portable telephones define an output range, and designers had better design telephones which can provide an output close to the allowable maximum output.
A radio frequency power amplifier (radio frequency power amplifier circuit) at the output stage of the transmission end of a cellular type portable telephone has a configuration in which the output is controlled by an APC (automatic power control) circuit and in which a gate voltage is controlled to provide an output as required for a call.
Since the power amplifier circuit has the best power source efficiency at the maximum output thereof, an abrupt reduction in the power supply efficiency occurs when the power amplifier circuit has a low output level. Therefore, the power supply efficiency is low when the telephone is used at a low output level, e.g., when it is used in the vicinity of a base station, which causes consumption of the battery at a high rate to shorten the life of the battery. This reduces call time per battery.
Conventional radio frequency power amplifiers suffer from deterioration of linearity and AM/AM characteristics (AM/AM conversion) at a low output (e.g., +5 dBm) because the gate bias of a power MOS at the final stage thereof is simply decreased.